The gas-phase olefin polymerization method using a fluidized bed has long been applied for commercial use. FIG. 1 is a schematic diagram showing a conventional fluidized bed reactor for use in a polymerization reaction in which polymerization of olefin is carried out. As illustrated in FIG. 1, the fluidized bed reactor 10 is divided into a reaction region A in which a polymerization reaction of olefin takes place, and a free region B positioned on the top of the reaction region A and having most of the solid polymer particles separated from the gas phase. In the reactor 10, the reaction region A is confined to a vertical cylinder section a. To maintain the fluidized bed of polyolefin produced in the reaction region A, a reactive gas stream is continuously fed into an inlet 11 formed in the bottom portion of the reactor 10. The unreacted monomers of the reactive gas stream exhaust through an outlet 13 positioned on the top portion of the reactor 10. Separated from the released reactive gas stream and cooled down, the residual particles are fed back to the lower part of the polymer layer through the inlet 11 in the bottom portion of the reactor 10. The polymer (i.e., the product) formed in the reactor 10 is continuously removed from the fluidized bed. In FIG. 1, the reference numeral 14 designates an inlet for feeding catalyst particles or a pre-polymer and the reference numeral 12 designates a gas distributor.
FIG. 2 is a schematic diagram showing another type of the fluidized bed reactor for use in a polymerization reaction in which a polymerization reaction of olefin occurs. The polymerization reactor illustrated in FIG. 2 is an internally circulating fluidized bed reactor 10 that is equipped with a draft tube 20 in a fluidized bed A to circulate solid (polyolefin) particles. The draft tube 20 partitions the internally circulating fluidized bed reactor 10 into two polymerization regions. The inside of the draft tube 20 forms a riser region in which the growing polyolefin moves upward under the fast fluidizing conditions, and the outside of the draft tube 20 becomes an annulus region in which the polyolefin after passing through the riser region moves downward with gravity. Passing through the annulus region, the polyolefin goes back to the lower part of the riser region. In this manner, the polyolefin is circulated to undergo the polymerization reaction while moving between the riser and annulus regions. As shown in FIGS. 1 and 2, the general fluidized bed polymerization reactor as used in a polymerization of olefin consists of a cylindrical base section “a” forming the reaction region A and a conical top section forming the free region B.
In the fluidized bed reactor 10 for use in a gas phase polymerization of olefin, it is necessary to prevent agglomeration of the product such as sheeting or caking (i.e., reactor contamination) on the wall or other parts of the reactor 10. There have been suggested different methods for the solutions to this problem. For example, U.S. Pat. No. 4,956,427 discloses a method of hardening aminosilicone through hydrolysis to coat the inner surface of the gas phase polymerization reactor, which method disadvantageously requires a long-term cessation of operation and takes high cost to apply a coating on the inside of the reactor. U.S. Pat. No. 3,984,387 describes a method of preventing the formation of locally overheated polymer particles by injecting an inert gas, such as nitrogen, helium, etc., together with a monomer gas into the polymerization reactor. This method may reduce the partial pressure of the monomers in the reactor but deteriorate the catalytic activity. In addition, U.S. Pat. Nos. 4,650,841 and 4,551,509 disclose a method of preventing fouling by using a deactivating agent to reduce the catalytic activity. U.S. Pat. No. 5,733,988 specifies the use of an alcohol-, ammonia-, or sulfur-containing material as an anti-fouling agent. U.S. Pat. No. 5,804,678 describes a method of preventing fouling by adding water, alcohol, or ketone. However, these methods are to deteriorate the catalytic activity for the sake of preventing agglomeration of particles, only to decrease the reactivity. U.S. Pat. No. 5,473,028 discloses a method of preventing fouling without reducing the catalytic activity by adding a supported alumoxane or a solid alumoxane into the reactor, which method involves an economical difficulty for commercial use because the alumoxane is very expensive.
On the other hand, many attempts have been made to solve the problem in association with the agglomeration of the products by improving the design of the reactor. For example, U.S. Pat. No. 4,003,712 discloses a vertical fluidized bed reactor having a cylindrical base section extending to a short conical section and an additional cylindrical base section with a cross section greater than that of the said cylindrical base section. The polymerization reaction occurs in the lower portion of the reactor, while the polymer particles are separated from the gas stream in the top portion of the reactor that forms a stable region. Further, WO 96/04322, EP 0301 872, EP 0 475 603, and EP 728 771 make an attempt to solve the problem regarding the agglomeration of the product based on the geometry of the reactor. Unfortunately, the reactors disclosed in those patents are susceptible to the caking of polymer particles in the transition region between the base and top sections, so a continuously reduced cross section is formed to cause a cessation of the polymerization reaction, which presents a need for washing the reactors. Further, U.S. Pat. No. 5,428,118 discloses a method of suppressing the sheeting or caking of the product or eliminating the caking particles by feeding an air stream in the tangential direction along the wall of the free region. This makes the structure of the reactor too complicated. The common feature of the above-specified methods is that the fluidized bed A is located in the cylindrical section “a” of the reactor.